How should I retouch videos (repaint areas)?
First of all:.

I do strongly suggest to not put the video as texture on a plane in 3D-space. If you wanna use video input, it's objectivly better to use the sequence editor of blender, where you can add your videos and overlay them with renderings.

Second:

I think you kinda overestimate the ambition of this stack exchange community, because explaining how to do this task is probably more complicated than doing it. I don't think you will get a fully-featured tutorial on how to do this as answer, cause it's just too much work.

There are, however already tutorials how to do this already online. I suggest the following guide: https://www.youtube.com/watch?v=CVPcT0dJmoY